Dark matter a dying theory?

I think the analogy is apt, but perhaps strained. The hypothetical Vulcan was merely difficult to detect. But there was no belief that it would be impossible to detect.It may well be that dark matter is impossible to detect other than through its gravitational effect. There may never be anything other than highly circumstantial evidence for dark matter. Unlike the case of Vulcan, every failure to detect dark matter may mean simply dropping one particular dark matter hypothesis.

No, Vulcan was supposed to be inside the orbit of Mercury. It could not possibly remain on the opposite side of the sun from us. Vulcan was thought to be too close to the sun to easily detect.Vulcan was only one of a number of possible explanations for Mercury's perihelion anomaly being investigated. Others included a fast rotating solar core and solar oblateness.